1. Field of the Invention
The present invention relates to a displacement detection apparatus that detects a displacement of a subject.
2. Description of the Related Art
There has conventionally been known a reflective-type optical encoder as a displacement detection apparatus that detects a displacement of a subject to be measured, such as a moving amount or rotation amount. The reflective-type optical encoder includes a light-emitting element, a scale that reflects light emitted from the light-emitting element and is displaceable relative to the light-emitting element, and a light-receiving element that receives light reflected by the scale.
The scale is provided with a reflective layer that is formed in a lattice structure for reflecting light. Therefore, the quantity of light received by the light-receiving element changes depending on the relative displacement of the scale. The displacement is detected based on a detection signal produced due to the change in the quantity of light at the light-receiving element.
Japanese Patent Application Laid-Open No. 2004-028862 discusses the reflective-type optical encoder described above. This application discusses that a glass or resin is generally used for a base of the scale. The subject of the invention in Japanese Patent Application Laid-Open No. 2004-028862 is such that, when incident light is reflected by the back surface of the base of the scale made of a glass or resin having no reflection layer formed thereon, this reflected light is incident on the light-receiving element as a noise component, which reduces an S/N ratio of the detection signal.
This application discusses the configuration (configuration 1) in which an antireflection film or light-absorbing film is formed on the back surface of the base, and the configuration (configuration 2) in which the back surface of the base is subject to surface-roughening to form a diffuse-reflection surface, for solving the subject.
In the configuration 1, the antireflection film is formed on the back surface of the base, so that the thickness of the scale is increased by the film. When the back surface of the scale is attached to the member whose displacement is detected, the antireflection film and the base might be peeled upon the attachment or after the attachment. In the configuration 2, the surface-roughening process is used, so that productivity is poor and production cost is unsatisfactory.
When a glass is used for the base of the scale as discussed in Japanese Patent Application Laid-Open No. 2004-028862, the thickness cannot be decreased, because the strength is insufficient. Therefore, the thickness of the scale is further increased. When a resin is used, the scale may be deformed due to the irradiated light, other members, or heat from external environment, because resin has a great thermal expansion coefficient. Therefore, the measurement precision is deteriorated.